1. Field of Invention
The invention relates to an optical analysis device according to the principle of radiation absorption with a housing having at least one radiation-permeable housing element, with at least one radiation source and a reflector assigned to it, with at least a first detector and a second detector and with an external reflector located outside of the housing. An absorption space is formed by the external reflector and the radiation-permeable housing element, wherein a measuring beam emitted by the first radiation source and the first reflector re-enters the housing after reflection on the external reflector.
2. Description of Related Art
Optical analysis devices of the type under consideration exploit the effect of radiation absorption by matter, which is penetrated by electromagnetic radiation. The electromagnetic radiation is generally broadband and covers at least the frequency range in which the substance to be detected acts to absorb radiation. Each substance to be detected shows a characteristic absorption spectrum. The emitted electromagnetic radiation, after passing through the absorption space in which the substance to be detected is located, is relatively strongly attenuated in certain, generally narrowband absorption regions relative to the radiated power of adjacent frequency ranges. While the substances present can be identified via recording of an absorption spectrum, it is also possible to draw conclusions about certain concentrations of the substance via the intensity of the relative attenuation in the absorption range.
Analysis devices of the type under consideration here are used especially for measuring toxic and explosive gases. Here, absorption in the infrared wavelength range characteristic for many gases is used. This absorption of gases and vapors is very specific to a material with respect to its spectral dispersion as well as the degree of absorption. For this reason, the gas-specific absorption is used for identification and determining concentration. Non-dispersive infrared (NDIR) analysis is used here and does not involve spectral decomposition of the emitted electromagnetic radiation. Instead, selective detectors are used which are sensitive only in a limited radiation range, specifically in the range in which the substance to be detected absorbs radiation. Here, optical filters having narrowband transmittance are used to select the radiation of an initially broadband light source, so that they correspond to the absorption of the gas to be measured. The intensity of the infrared radiation is then detected with pyroelectric detectors or thermopiles and analyzed by subsequent electronics, usually a microcontroller.
It can be easily imagined that, when using only one detector, almost no conclusions can be drawn about the actual concentration of the substance which is to be detected, if it is possible for attenuation of the measuring means to be caused in some other way, for example, by the presence of interfering gases and other contaminants in the absorption space. To the same degree, for example, aging-induced intensity attenuation of the radiation source also cannot be detected with only one detector. To compensate for these effects, use of at least two detectors is therefore known in the prior art, of which one detector is sensitive in the absorption range of the substance to be detected and the other detector is sensitive in the frequency range in which absorption by other substances is not possible (reference detector and measuring detector). Certain effects, which adulterate the measurement, can be compensated for by the signal obtained from the measuring detector being referenced to the signal obtained from the reference detector.
Such dual-wavelength systems are used industrially for the protection of people and facilities from toxic and/or explosive gases and vapors. They are often integrated into portable devices, so that it comprises a small and energy-saving system. Just as often, there is a need to be able to simultaneously measure multiple optical detectable gas components with one single device. This exists, for example, in monitoring the danger of explosion from hydrocarbons and the simultaneous measuring of toxic carbon dioxide.
It is known in the prior art to use two independent apparatus in one device. German Patent Application DE 196 04 167 A1 proposes such a sensor device for detecting gas concentrations, which has multiple selective radiation detectors that are arranged annularly at different distances around a broadband radiation source. The arrangement is relatively space consuming and expensive due to the annular arrangement of the detectors having more or less each straight-line, unfolded radiation paths. Only a small dihedral angle section of the radiation source reaches the detectors. This makes high-energy radiators necessary to obtain a good signal to noise ratio.
A construction with two detectors and radiation paths of differing lengths is also proposed according to KR 000190693063 BA. In this arrangement, too, only a small portion of the radiation reaches the detectors, so that the result is a high consumption of energy.
Moreover, there is a problem in that only weakly absorbing components require a longer absorption path compared to strongly absorbing components in order to maintain an identically strong signal at the same material concentration. Depending on the measuring components and the concentration range to be detected, an ideal combination of measuring wavelength range and absorption path always results.